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5V7855SWFR 데이터시트(PDF) 5 Page - Integrated Device Technology

부품명 5V7855SWFR
상세설명  EEPROM Programmable PLL Die for LVCMOS Crystal Oscillator
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제조업체  IDT [Integrated Device Technology]
홈페이지  http://www.idt.com
Logo IDT - Integrated Device Technology

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IDT5V7855-DPK REVISION A MARCH 11, 2010
5
©2010 Integrated Device Technology, Inc.
IDT5V7855 Data Sheet
EEPROM PROGRAMMABLE PLL DIE FOR LVCMOS CRYSTAL OSCILLATOR
The fractional multiplier is determined from the phase detector
frequency and the VCO frequency using the following equations.
Calculate the fractional multiplier decimal value:
Calculate the fractional multiplier register binary value:
MBIN = MDEC x 16384
Example 1:
The crystal frequency is 27MHz and the desired output frequency is
166.66667MHz.
1.
With a crystal frequency of 27MHz, the appropriate
prescaler value is 1 and the associated phase detector
frequency would be: 27MHz ÷ 1 = 27MHz.
2.
For an output frequency of 166.6667MHz, the output
divider which can be used with a VCO frequency in the proper
170MHz to 340MHz range is 1, and the resulting
VCO frequency is: 166.6667MHz x 1 x 2 = 333.33MHz.
3.
The fractional multiplier is calculated as the VCO frequency
divided by the phase detector frequency or
333.33MHz ÷ 27MHz = 12.345679.
4.
The MBIN is found by 12.345679 x 16384 = 202271.6 and is
rounded to 202272.
5.
Due to the rounding, the actual MDEC value will be 12.3457
and the actual output frequency will be:
FOUT = 166.66699MHz
(NOTE: This is an error of only 1.94ppm.)
Example 2:
The starting crystal frequency is 48MHz and the desired output
frequency is 25MHz.
1.
With a starting crystal frequency of 48MHz, the appropriate
prescaler value can be either 1, 2 or 3 and the associated
phase detector frequency would be either:
48MHz ÷ 1 = 484MHz,
48MHz ÷ 2 = 24MHz or,
48MHz ÷ 3 = 16MHz.
2.
For an output frequency of 25MHz, the output dividers
which can be used with a VCO frequency in the proper
170MHz to 340MHz range are: 4, 5, or 6, and the resulting
VCO frequencies is: 200MHz, 250MHz, or 300MHz.
3.
The fractional multiplier is calculated as the VCO frequency
divided by the phase detector frequency. So, for the
possible combinations identified, the resulting fractional
multiplier could be one of nine possibilities.
Table 3D. Fractional Multiplier Table
4.
With the different MDEC values to choose from, how does
one select the ‘best’ value? In general, selecting a fractional
value which reduces the ppm error is the first selection
criteria. This applies to MDEC values which have the fewest
non-zero decimal places or MDEC values which have large
integer values. This criteria would eliminate the
MDEC = 4.16666, 5.208333, 8.33333, and 10.4166666
options. After the ppm error selection is made and if there are
still other options, generally the higher VCO frequency will
provide better performance. In this example, the higher VCO
frequency corresponds with D = 6 and the associated P and
MDEC values of 1 and 6.25, 2 and 12.5, or 3 and 18.75. Lastly,
the higher phase detector frequency generally provides better
performance. So for this example, the condition of P = 1,
MDEC = 6.25, and D = 6 would be the preferred choice.
5.
The MBIN is found by: 6.25 x 16384 = 102400. (No rounding is
necessary.)
6.
Verifying the values
M
DEC
F
VCO
F
PHASE
DET
-----------------------------------
=
F
OUT
F
XTAL x
M
DEC
Px Dx 2
()
---------------------------27MHz x
12.3457
1x1 x 2
---------------------
=
=
P = 1
FPHASE_DET
(48MHz)
P = 2
FPHASE_DET
(24MHz)
P = 3
FPHASE_DET
(16MHz)
D = 4
FVCO =
200MHz
M
DEC =
4.16666
M
DEC=
8.33333
M
DEC =
12.5
D = 5
FVCO =
250MHz
M
DEC =
5.20833
M
DEC=
10.41666
M
DEC =
15.625
D = 6
FVCO =
300MHz
M
DEC =
6.5
M
DEC =
12.5
M
DEC=
18.75
200MHz
48MHz
-----------------------
200MHz
24MHz
-----------------------
200MHz
16MHz
-----------------------
250MHz
48MHz
-----------------------
250MHz
24MHz
-----------------------
250MHz
16MHz
-----------------------
300MHz
48MHz
-----------------------
300MHz
24MHz
-----------------------
300MHz
16MHz
-----------------------
F
OUT
F
XTAL x
M
DEC
Px Dx 2
()
---------------------------48MHz x
12.5
2x 6 x 2
---------------------
=
=


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